Galaxy Design Leggings Free Mockup
Creatsy from Creative Market created a simple yet true to life Galaxy Design Leggings Free Mockup wherein you can drag and drop smart objects into the PSD. You can design from scratch and try to incorporate elements into the leggings.
But what I can really suggest is that create a design/color scheme already and make sure that it reflects your personality, then you can add it as a smart object into the mockup. People would also love to see colors but everything should be on brand.
The main factor that you should give time to think is your color scheme, make sure that people will know what its theme is with just a glance. Customers will never forget your brand if you launch a product that will appeal to them in an extravagant yet striking way.